Increased power density is a continuing goal of modern power supply design. High power density is particularly crucial in applications wherein the allocated space for the power supply relative to the power output is restricted. In addition to being highly compact, the power supply should also be efficient to limit heat-creating power dissipation. Illustrative applications for a high density power supply include an off-line power supply used to power a laptop computer or a power supply module for a telecommunication system employing an Integrated Services Digital Network ("ISDN").
One alternative to decrease the size of the power supply relative to the power output is to increase the operating frequency of the power supply. However, there are upper practical limits to the operating frequency for the components of the power supply. Therefore, a power supply topology capable of offering higher efficiency at elevated switching frequencies is required to ease the task of thermal management at higher power density.
Employing a current doubler rectifier in a converter results in a particularly attractive topology for powering high current loads for applications such as those mentioned above. This topology reduces the current density in critical secondary-side magnetic components of the converter and offers a twofold increase in the equivalent output frequency without concomitantly increasing the operating frequency and the associated switching losses of the converter.
The first known reference to the current doubler rectifier is found in Elements of Electrical Engineering, A Textbook of Principles and Practice, by Arthur L. Cook, John Wiley & Sons, Inc., 1924. Presently, the current doubler rectifier is more readily referred to as a "hybridge" rectifier circuit. Such circuit is more definitively disclosed in U.S. Pat. No. 4,899,271 to Seiersen, issued on Feb. 6, 1990, entitled "Power Supply Circuit" and incorporated herein by reference. The circuit of Seiersen comprises a transformer with a pair of rectifier diodes connected to an output load. The first electrode of the rectifier diodes is connected to the secondary winding of the transformer and to an inductor located between the rectifier diodes and the output of the power supply. The second electrode of the rectifier diodes is connected through a capacitor to the output of the power supply. The secondary winding of the transformer draws current through one of the diodes via one inductor while the other inductor draws current through the same diode. This operation allows the secondary winding of the transformer to be partially bypassed such that both currents simultaneously contribute to the output current.
As hereinafter described, the hybridge rectifier circuit yields several unique advantages. First, the transformer structure is greatly simplified because there is no need for center-tapping the secondary winding, thereby eliminating one high current termination thereon. Also, the secondary winding and its terminations carry approximately half the current of their full wave equivalents. Moreover, the hybridge rectifier circuit offers a lower turns ratio and finer steps in the transformer, resulting in reduced leakage inductance for a more efficient transformer. Further, the control and operation of the primary side of the converter, including the duty cycle of the power switches, remains unchanged. Therefore, the hybridge rectifier circuit can easily be added to an existing front-end converter system design.
Second, while the hybridge rectifier circuit requires an additional inductor, both output inductors carry only half of the output current and operate at half the operating frequency of their full wave counterparts. These individual inductors are, thus, less bulky, leading to a more flexible design. Finally, the power dissipation across the converter is more evenly distributed thereby easing thermal management and packaging allowing for a more compact overall converter design. Moreover, the current waveforms of the individual inductors are typically out of phase resulting in a partial or full cancellation of the ripple components in the common output impedance of the converter. The resulting reduction, or better yet cancellation, of the output ripple current renders a more efficient converter. Thus, for the aforementioned reasons, the hybridge rectifier circuit provides a viable solution in high output current applications.
As previously mentioned, the hybridge rectifier circuit is compatible with several front-end converter designs including a conventional bridge-type power train. One main attribute of the bridge-type power train is that the transformer windings are driven with power switches providing bipolar voltages with dead time in between. At a 100% duty cycle of the power switches, a condition exists whereby the ripple currents in the two output inductors are equal and opposite, leading to full cancellation and a zero-ripple current at the output capacitor of the converter. The zero-ripple current condition only occurs at a single input voltage for a given power train and output voltage of the converter. For bridge-type converters, the zero-ripple current point can only occur at the lowest input voltage; thereafter, the capacitor ripple current continually increases as the input voltage increases. Thus, the filter inductors and capacitors at the output of the converter must be oversized for the worst-case operating condition, thereby diminishing the benefit derived from converters operating at the zero-ripple current condition. Moreover, operating the converter at the 100% duty cycle in an attempt to cancel out the output ripple current is not a practical operating point for the power supply. Thus, other converter topologies employing the hybridge rectifier circuit are more attractive.
The hybridge rectifier circuit can also be employed with a converter employing an active clamp front-end. This power train also includes power switches that drive the transformer with bipolar voltages, but the dead time exhibited by the bridge-type power train is not present. As a result of the lack of dead time, the frequency of the ripple current encountered by the output capacitor is equal to the switching frequency of the power train. This converter type also experiences a zero-ripple current operating point; however, in opposition to the bridge-type converter where the zero-ripple current condition occurs at the 100% duty cycle, the active clamp converter experiences the zero-ripple current condition at a 50% duty cycle. Operating the converter at a 50% duty cycle is a more realistic operating point for the converter.
It is possible, therefore, to design the converter centered at an input voltage range encompassing the zero-ripple current point, thereby minimizing the worst case ripple current at the output of the converter. While the active clamp converter moves the zero-ripple current condition off the 100% duty cycle, it is undesirable to design the power train at the input voltage relating to the 50% duty cycle because of the stress placed on the components at off design input voltages. Thus, although the present day active clamp converter is an attractive power supply for high current loads, it is limited in its application. More specifically, the active clamp converter suffers from the shortfalls of excess stress on the converter components or excess ripple current in the output depending on the selected design of the converter. Accordingly, a system to reduce the output ripple current independent of the operating condition of the converter is necessary.
There have been attempts in the prior art to deal with ripple current cancellation in converters. For instance, Loftus, in U.S. Pat. No. 5,353,212, issued Oct. 4, 1994, entitled "Zero-Voltage Switching Power Converter with Ripple Current Cancellation" and incorporated herein by reference, discloses an integrated magnetic arrangement that provides ripple current cancellation at selected operating points of the converter. In an illustrative embodiment of Loftus, the inductances of the integrated magnetic device are selected such that the output currents are equal, but out of phase for a given load; this condition provides for the complete cancellation of the ripple current at the output of the converter. While Loftus teaches a technique to reduce the ripple current of a converter, Loftus' technique requires the combination of an integrated magnetic structure with a center-tapped rectifier circuit in the secondary side of the converter. In Loftus, integrating the magnetic devices is required to perform ripple current cancellation.
Hypothetically, if the center-tapped rectifier circuit disclosed in Loftus includes a transformer with a pair of discrete inductors, then ripple cancellation cannot be performed by the discrete inductors and the output capacitor must be oversized to handle the ripple current in the output of the converter. Thus, Loftus' technique is limited in its application to converters powering high current loads because the converter must incorporate either an integrated magnetic device or an oversized capacitor into the design to achieve ripple cancellation both of which add size and complexity to the design of the converter.
Accordingly, what is needed in the art is a circuit and method to achieve a zero-ripple current condition in the output of a power converter without adding size and complexity to the rectifier circuit of the converter.